I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for providing high speed data in a wireless cellular communication environment.
II. Description of the Related Art
As wireless communication technology has advanced, an increase in the demand for high speed data services in a wireless environment has grown dramatically. The use of code division multiple access (CDMA) modulation is one of several techniques for providing digital wireless transmission that is well suited for the transmission of digital data. Other methods of digital wireless transmission include time division multiple access (TDMA) and frequency division multiple access (FDMA).
However, the spread spectrum modulation technique of CDMA has significant advantages other digital modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. The method for providing digital wireless communications using CDMA modulation was standardized by the Telecommunications Industry Association (TIA) in TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereafter IS-95).
The current wireless communication systems can only accommodate relatively low transmission rates. In addition, most current wireless communication systems have not been optimized for the transmission of digital data, but rather have been optimized for the transmission of speech information. Therefore, there is a need in the industry for a method of providing high speed digital data in a wireless environment.
The present invention is a novel and improved method and apparatus for transmitting digital data in a cellular environment. In the present invention, adjacent cells of the cellular system are prevented from simultaneously transmitting data. Thus, if a first base station on one side of a cell boundary is transmitting, then a second base station on the other side of the cell boundary is silent throughout the transmission period of the first base station. Because the noise from transmissions of adjacent cells is a primary source of interference, the transmission rate of power limited base stations can be dramatically increased when the noise from adjacent cells is eliminated.
In the present invention, all transmissions from a base station are transmitted at a fixed power level and the transmissions to each subscriber station in a cell are transmitted in non overlapping bursts. Thus, when a base station is transmitting, its transmissions are directed to one subscriber station within the cell, allowing the full amount of available power to be used to transmit data to that subscriber station which maximizes the available data rate to the subscriber station.
For the sake of clarity, it should be noted that two separate but related rates are referred to herein. One is the information rate which refers to the rate of user generated information bits. The second is the transmission rate which is the rate of bits transmitted over the air.
When transmissions are made at a fixed power level, the amount of information that can be transmitted between the base station and the subscriber station varies with link budget factors which are well known in the art. The most significant link budget factor in a wireless communication system is the path loss between the base station and the subscriber station. The path loss is strong function of the distance between the base station and the subscriber station.
In the present invention, the transmissions to each subscriber station are made at a fixed transmission power level. However, the information rate of transmitted signals differs depending the distance between the subscriber station and the base station. In the first exemplary embodiment, the information rate of transmissions to a subscriber station is determined by selecting an encoding rate for the transmitted signal while holding the transmission rate constant. In the second exemplary embodiment, the information rate of transmissions to a subscriber station is determined by selecting a modulation format for the transmitted signal which directly changes the transmission rate of transmission to a subscriber station.